Correct Code of Conduct copyright and license.

As pointed out by @tannewt in
adafruit/cookiecutter-adafruit-circuitpython#75,
the CoC is derived off the Contributor Covenant, and is also released as
CC-BY-4.0 (see https://github.com/adafruit/Adafruit_Community_Code_of_Conduct).

Author: Flameeyes

CODE_OF_CONDUCT.md

@@ -1,9 +1,9 @@
 <!--
+SPDX-FileCopyrightText: 2014 Coraline Ada Ehmke
 SPDX-FileCopyrightText: 2019 Kattni Rembor for Adafruit Industries
 
-SPDX-License-Identifier: MIT
+SPDX-License-Identifier: CC-BY-4.0
 -->
-
 # Adafruit Community Code of Conduct
 
 ## Our Pledge